Ultra-high-frequency tuning apparatus



Jan. 16, 1945. R, HARVEY ET AL 2,367,576

ULTRA HIGH FREQUENCY TUNING APPARATUS Filed April 1, 1942 1 16. 1. FS'G. Z.

MEG/ICYCLES 600 3nventors Robert L. Harvey 1% Harold G. Fisher k Lia-M Patented Jan. 16,

, .assasvc ULTRA-HIGH-FREQUENCY TUNING APPARATU Robert L. Ham, Collingswood, and Harold G.

Fisher, Camden, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application April 1, 1942, Serial No. 437,104

2 Claims.

This invention relates to tuning apparatus and especially to variable tuning apparatus for use in ultra-high frequency (say, upwards of 250 megacycles) signaling systems.

The principal object of the present invention is to provide a compact yet reliable substitute for concentric line and other relatively ponderous apparatus heretofore employed in the tuning or tank circuits of ultra high frequency signaling systems.

The invention will be described in connection with the accompanying drawing, wherein:

Fig. 1 is a plan view of metal structure possessing inductance and capacitance and which will be referred to in explaining one of the problems solved by the present invention;

Fig. 2 is a front elevation and Fig; 3 a. side elevational view of a tuning device which is constructed, in accordance with the principle of the present invention, in the form of a metal ribbonlike spiral or volute; I

Figs. 4 and 5 are plan views of different spiralshape tuned circuits within the invention and showing, diagrammatically the electrical equivalents of each;

Fig. 6 is a circuit diagram showing one of the numerous possible applications of the tuning de- .vice of the invention; and

Fig. 7 is a chart on which the Q" or factor of merit of a tuned circuit within the invention, is plotted.

It is well known to those skilled in the design of electron discharge tubes that there is a point in the frequency spectrum at which it becomes necessary to consider the effect of electron transit time. Thus, if the 'interelectrode space traversed by the electrons is so great that the time it takes an electron to traverse this space is longer than the period of the oscillations impressed upon the tube, the electrons will not impinge the target in phase with the oscillations. power losses, a general decrease in efiiciency and, at very high frequencies, total inoperability. The present invention is predicated upon the discovery that a similar situation arises in connection with a design of tuned circuits for use at ultra high frequencies. By way of example, let us assume that the more or less conventional tuned circuit-of Fig. 1 is responsive to a source of ultra high frequency oscillations (not shown) and that at a given half cycle the capacitor plates Cl--C2 have assumed opposite charges, as indicated by the plus and minus symbols. Now, in agreement with the electron theory, upon the following half cycle the electrons a and b This results in on the plate C1 and which give rise to the negative charge on said plate will flow to the opposite plate C2. However, it will be apparent that the electron a adjacent to ,the end of the plate C1 will have a greater distance to travel than the electron b (which is located adjacent to the point where the wire L is joined to the plate C1) and hence will not arrive at the plate C2 at the same instant as electron b. This difference in transit time of the electrons (a and b) in a tuned cir- ,.cuit is manifest by a low Q, or factor of merit.

( resistance of about 200 at frequencies of the order of 250 to 500 megacycles per second.

v The present invention contemplates a tuned circuit wherein different transit distances which might give rise to differences in electron transit time (and hence phase differences resulting in low Q) are compensated for by subjecting the electrons to different accelerating potentials so that their transit times are substantially equalized.

. This desired result is achieved in accordance with the invention by means of a conductive spacewound ribbon-like spiral or helix possessing inductance and distributed capacitance of a desired value and normally subjected to a progressive potential gradient such as to compensate for differences in electron transit distances.

Referring now to Figs. 2 and 3, here H designates' a space-wound 'ribon-like fiat spiral (which will be understood to possess inductance and distributed capacitance of a value calculated to cause it to respond, electrically, to a frequency of 250 megacycles or higher) and which is secured at its inner end to a post or riser P on an insulating base B. If desired the ribbon H may be constituted of phosphor bronze, silver plated steel, or other spring metal, in'which case its free end (either its outer or its inner end) may be provided with an adjusting mechanism, such, for example, as a winch W and cord S, for altering the space between the turns of the spiral, whereby to change its distributed capacitance and hence its tuned frequency.

Ordinarily, the spiral H need not comprise more than two full turns. Thus, in the spiral H shown in Figs. 2 and 4, the outer tum overlaps but onehalf the inner turn and in the spiral H shown in Fig. 5, the outer turn overlaps one full inner turn. In ,this connection it may be notedthat where, as in Fig. 5, the outer end of the spiral is grounded, the outer turn comprises a shield which inhibits radiation from so much of the inner turn as it covers.

The capacitance of the tuned spiral H of Fig. 4 and of the tuned spiral H of Fig. 5 comprises only "distributed" capacitance. Thus, if it is assumed that there is a phantom capacitor between points 0 and 2, another between point i and 3, and another between points 2 and 4, then the circuit equivalents of these spirals may be illustrated in conventional symbols by a coil L whose tapped turns are designated 0 to 4, inclusive, and wherein miniature capacitor Ml M2 and M3 are shunted between points 0-2, |--'3, and 2-4, respectively. It is, of course, apparent that this analogy is not perfect since the distributed capacitance in the spirals H and H would comprise an infinite number of such phantom capacitors.

As shown in Fig. 6, two spirals HI and H2 may be connected in series to provide a balanced transmitter or receiver system wherein the tuning system is matched with a transmission line T. The inductance between points p and p in this case is so chosen that; the impedance measured between the points matches or terminates the line T. When these spirals HI and H2 are connected between the plate P and grid G ot a vacuum tube oscillator V, maximum output is achieved by varying the position of the plate and grid connectors in opposite directions along the spirals Hi and 1-12. Obviously, another type of tube, e. g., a detector or amplifier, may be substituted for the oscillator V of Fig. 6 or, if desired, a crystal detector may be employed. Where, as in Figs. t and 5, the tuner comprises a single spiral, the coupling between the spiral and the apparatus with which it is used may comprise a single connector X (Fig. 4) or it may be of the inductive or capacitive type, as desired.

Fig. 7 comprises a chart showing the factor of merit or Q" (the ordinate) plotted against frequency (the abscissa) of a tuning device constructed in accordance with the principle of the invention and similar to the one shown in Fig. 2. It will be observed that this device exhibited a "Q of approximately 1100 to 1300 over a tuning range of from 250 to 475 megacycles. In this case the length (expanded) of the metal ribbon comprising the spiral was approximately 3 /2 inches, and its width approximately of an inch. The same or higher Q may be achieved with smaller spirals at frequencies upwards of 1000 megacycles.

As previously indicated the high i'actor oi merit exhibited by the tuning devices or the present invention may be attributed to the tact-that the electrons flowing from point to point along the spiral are normally subjected to a progressively increasing potential gradient; that is to say, the electrons which traverse the high potential portion of the spiral travel at much higher speeds than those whose flow is limited to the low potential end and this fact compensates (in terms of electron transit time) for the differences in their transit distances. The invention, however, is not limited to this particular theory of operation.

The invention is susceptible of various modifi cations. By way of example: instead of the hat or watch spring spiral of the drawing, an elongated (like the thread of a screw) edge mounted helical ribbon may be used. In this latter case variations in the distributed capacitance may be effected by compression and expansion of the helix along its long axis or pole, instead of in the lateral direction. Obviously, an adjusting mechanism comprising a screw may be substituted for the cord and winch of Figs. 2 and 3.

Various other modifications of the invention will suggest themselves to those skilled in the art. .It is to be understood therefore that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

We claim:

1. A circuit for ultra high frequencies comprising a ribbon-like conductor space-wound in the form of a relatively flat spiral consisting of substantialiy no more than two turns and possessing inductance and distributed capacitance of a value to tune it to a frequency of the order of at least substantially 250 megacycles, an end of said conductor comprising a high potential terminal of said circuit, said high potential terminal being free from an impedance load, and a connector for a utilization circuit connected to said conductor at a potential point intermediate its ends. 2. The invention as set forth in the preceding claim and wherein means are provided for effectively varying the spacing between the turns of said ribbon-like conductor whereby to vary the frequency to which it is tuned by a predominant change in the distributed capacitance thereof.

ROBERT L. HARVEY.

HAROLD G. FISHER. 

